1. Field of the Invention
The present invention relates to a thermal head which is mounted on a thermal printer and energized and heated in accordance with printing information to perform a desired printing.
2. Description of the Related Art
In general, a thermal head mounted on a thermal printer includes a plurality of heater elements arranged linearly on one substrate. Such a thermal head is used for performing printing with coloring a heat-sensitive recording paper or with transferring ink to a plain paper through an ink ribbon by selectively energizing and heating each of the heater elements in accordance with desired printing information.
FIG. 3 shows a conventional thermal head. Referring to FIG. 3, a glaze layer 2 composed of glass and the like functioning as a heat accumulating layer is formed on an insulating substrate 1 which is composed of ceramic such as Al.sub.2 O.sub.3. The top surface of the glaze layer 2 at a portion corresponding to the position of a heating portion is so formed as to have a circular arc sectional configuration. Heater resistive elements composed of Ta.sub.2 N and the like are adhered to the surface of the glaze layer 2 by vapor deposition or sputtering. Then, the elements are etched to become a plurality of heater elements 3 responsive to the dot numbers arranged linearly on the top surface of the glaze layer 2. A common electrode 4 to be connected to each of the heater elements 3 is formed on one side of the heater elements 3, and an individual electrode 5 energizing separately each of the heater elements 3 is connected to the other side of the heater elements 3, respectively. These common electrode 4 and individual electrode 5 are composed of Al, Cu or a metal, and adhered to the glaze layer 2 by vapor deposition or sputtering, and then patterned into desired shapes by etching.
Furthermore, a protective layer 6 having a thickness of about 5-10 .mu.m is formed on the surfaces of the heater elements 3, the common electrode 4 and the individual electrode 5 so as to protect the glaze layer 2, heater elements 3, the common electrode 4 and the individual electrode 5. This protective layer 6 covers entire surfaces except terminal portions of the electrodes 4 and 5. The protective layer 6 includes an oxidation-resistant layer 7 having a thickness of about 2 .mu.m composed of SiO.sub.2 or the like for protecting the heater elements 3 from deterioration due to oxidation, and a wear resisting layer 8 having a thickness of about 3-8 .mu.m composed of Ta.sub.2 O.sub.5 or the like for protecting the heater elements 3, the common electrode 4 and the individual electrode 5 laminated in this turn. The oxidation-resistant layer 7 and the wear resisting layer 8 are sequentially formed by vapor deposition or sputtering.
In a thermal transfer printer using the thermal head as described above, a desired printing is performed by selectively energizing and heating the individual electrode 5 of the heater elements 3 based on desired printing signals to fuse the ink of the ink ribbon and transfer to a paper with the thermal head being pressed into contact with the paper through the ink ribbon. In a thermal printer using the thermal head as described above, a desired printing is performed by selectively energizing and heating the individual electrode 5 of the heater elements 3 based on desired printing signals to color the heat-sensitive recording paper with the thermal head being directly pressed into contact with the paper carried onto a platen.
In such a thermal head as described above, by a combination of the glaze layer 2 of low thermal conductivity and the substrate 1 of high thermal conductivity composed of Al.sub.2 O.sub.3, electric power efficiency and printing properties are balanced utilizing heat accumulating effect of Joule heat generated at the heater elements 3. In other words, since the time constant for cooling the heater elements 3 is prolonged due to heat accumulating effect of the glaze layer 2, deterioration of printing quality such as tailing, bleeding and margin stain, and dot omission due to overheating of the heater elements 3 will occur. Thus, in consideration of electric power efficiency and printing properties, the thickness of the glaze layer 2 is controlled in accordance with use conditions thereof. Usually, the thickness of the glaze layer 2 is about 30-60 .mu.m.
In recent years, with an increasing need for a printer capable of high-quality printing and high-speed printing due to high definition, a thermal printer with printing resolution of 400 dpi and printing speed of 100 cps has become practical. In this thermal printer, energizing is controlled with a very short pulse width such as 300 .mu.s or less of a driving cycle of the heater elements 3. And, high definition and speeding-up of the printing tend to be further advanced.
In such a thermal printer for realizing high definition and high-speed printing, the printing quality is deteriorated by intensive heat accumulation of the thermal head. Thus, the thickness of the glaze layer 2 is reduced to about 30 .mu.m, and energizing time to the heater elements 3 is corrected with electrical means using LSI for correcting heat history so that temperature rise of the thermal head due to heat accumulation is closely controlled.
However, when high definition and speeding-up of the printing speed are further advanced, it is difficult to prevent deterioration of the printing quality due to heat accumulation of the thermal head by only such technique as described above thus, a technique which can thoroughly solve the problem of heat accumulation is demanded.
In a control of energizing at a very short pulse width such as such as 300 .mu.s or less of a driving cycle of the heater elements 3, for obtaining a desired printing quality, the peak temperature of the heater elements 3 of the thermal head must be increased to obtain a predetermined printing energy. For example, when environmental temperature at the time of printing is low such as 5.degree. C., high energy must be applied to the thermal head to perform printing, and the temperature increases together with the influence of the heat accumulation higher than about 700.degree. C. of which the glaze layer 2 and the heater elements 3 can withstand. As a result, the glaze layer 2 is fused or undergoes a thermal deformation, or electrical resistance value of the heater elements 3 is changed so that the thermal head cannot be used for the high-speed printing in a low-temperature environment.
Furthermore, since the heater elements 3 composed of cermet materials such as Ta-SiO.sub.2 and the like has properties such that the sheet resistance value thereof is reduced approximately by half when subjected to a high-temperature vacuum annealing treatment. Thus, although the high-temperature vacuum annealing treatment at a temperature higher than that of the actual use is essential to the heater elements 3, the heater elements 3 cannot be subjected to the high-temperature vacuum annealing treatment because the glaze layer 2 can withstand a low temperature as described above.
In addition, the glaze layer 2 composed of ceramic such as a glass or the like has low elastic modulus. Thus, when the terminals of the individual electrodes are connected to the connecting terminals of FPC by a solder, the glaze layer 2 cannot withstand a thermal stress due to contraction of a solder plating when cooled to be solidified, and a part of the glaze layer 2 is torn off and chipped.